Originally, digital broadcasting was mainly utilized in satellite broadcasting. However, in recent years, there has been a growing tendency of utilizing digitization in terrestrial broadcasting. In the terrestrial broadcasting, the waveform equalization technique for reducing the transmission line distortion is essential. Hereinafter, a prior art waveform equalization controller in the ground wave digital broadcasting will be described by using a DTV (Digital Television) system utilizing an 8-value VSB (Vestigial Side Band) modulation technique adopted in the U.S. as an example.
FIG. 19 is a block diagram illustrating a structure of the waveform equalization controller in the DTV system.
In FIG. 19, the prior art waveform equalization controller comprises a waveform equalizer 101 for receiving an input signal 100S and outputting an output signal 101S which is obtained by subjecting the input signal 100S to waveform equalization, an error estimation unit 1901 for estimating the error of the output signal 101S and outputting an error signal 1901S, and a coefficient updating amount calculation unit 103 for calculating a tap coefficient updating amount 106S based on a step size 104S, a coefficient updating data 105S to be used for the coefficient update and the error signal 1901S outputted from the error estimation unit 1901. The error estimation unit 1901 comprises an output error estimation unit 1502 for receiving the output signal 101S and outputting an output error signal 1501S, and a coefficient updating error generation unit 1902.
FIG. 20 is a diagram illustrating a structure of a signal format according to the DTV system. As shown in FIG. 20, the signal format according to the DTV system is composed of areas including a data signal 3101 such as video and audio, areas including a field synchronous signal 3102, and an area including a segment synchronous signal 3103.
FIG. 21 is a diagram illustrating a structure of the field synchronous signal 3102. As shown in FIG. 21, the field synchronous signal 3102 includes a PN511 signal 3201, three PN63 signals 3202, and a control signal 3203. The difference between the field synchronous signal #2 and the field synchronous signal # 1 is only that the second value in the PN63 signal 3202 is reversed. Values on the lefthand side of FIG. 21 (+7, +5, +3, +1, −1, −3, −5, −7) show 8 kinds of values which are taken on in the 8-value VSB modulation technique. One field of this DTV signal is composed of 832 symbols and 313 segments. The PN511 signal 3201 is obtained by generating a polynomial: PN511=(X^9)+(X^7)+(X^6)+(X^4)+(X^3)+(X+1) (in this polynomial, “A” shows the power) and is represented by Pre-load 010000000 (a binary representation of the initial value). The PN63 signal 3202 is obtained by PN63=(X^6)+(X+1) and is represented by Pre-load 100111. The PN511 signal 3201 is composed of 511 symbols. Each of the PN63 signals 3202 is composed of 63 symbols. The control signal 3203 is composed of 128 symbols. Accordingly, the entire field synchronous signal 3102 is composed of 828 symbols.
Next, the operation of the prior art waveform equalization controller is described with reference to the drawings. As shown in FIG. 19, in the waveform equalization controller according to the DTV system, the waveform equalizer 101 performs the waveform equalization to the DTV signal which has been subjected to the 8-value modulation (the input signal 100S) so as to reduce the distortion, and generates the output signal 101S. then, the output error estimation unit 1502 generates the error between the output signal 101S and the most likely one of the 8 kinds of symbol values as the output error signal 1501S. FIGS. 22(a) and 22(b) show the relationship between the output error signal 1501S and the error signal 1901S. The output error signal 15018 shown in FIG. 22(a) is converted by the coefficient updating error generation unit 1902 into the error signal 1901S as shown in FIG. 22(b) according to the value of the output signal 101S based on the Stop&Go algorithm described in “Blind Equalization and Carrier Recovery Using a “Stop-and-Go”, in Decision-Directed Algorithm IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. COM35, NO.9, SEPTEMBER 1987, pp. 877-887”. The coefficients updating amount calculation unit 103 generates the tap coefficient updating amount 106S based on a LMS (Least Mean Square) algorithm by using the error signal 1901S, the step size 104S and the coefficient updating data 105S.
The LMS algorithm is the algorithm of performing the n-th update of tap coefficient Ci of tap i in a transversal filter in the waveform equalizer 101 based on the following Expression 1.Ci(n+1)=Ci(n)−α×e(n)×di(n)  (Expression 1) 
Here, α denotes the step size 104S, e(n) denotes the error signal 1901S, di(n) denotes the coefficient updating data 105S, and −α×e(n)×di(n) denotes the tap coefficient updating amount 106S. Here, the coefficient updating data 105S is a data in tap i at the n-th updating.
There are two methods for generating the output error signal 1501S in the output error estimation unit 1502. One of the methods is capturing the PN511 signal 3201 and the PN63 signal 3203 included in the field synchronous signal 3102 in the DTV signal, comparing the signals with ideal values of the PN511 signal 3201 and the PN63 signal 3202, and outputting the error thereof as the output error signal 1501S.
The other method is comparing a data signal 3101 which is inserted in the data period of the DTV signal with the nearest fixed value of the 8-value VSB modulation, and outputting the error thereof as the output error signal 1501S.
Since the 8-value VSB modulation technique is used in the example described herein, the DTV signal has 8 kinds of values (+7, +5, +3, +1, −1, −3, −5, −7) as shown in FIG. 21. However, when the DTV signal which has been subjected to the 8-value VSB modulation is received, the DTV signal is generally quantized in about 10 bits. For example, in the case where the DTV signal which has been quantized in 10 bits does not have any distortion, only 8 kinds of fixed values can be taken on for the values which are represented by 10 bits (0 to 1023). The output error estimation unit 1502 calculates the error for the field synchronous signal 3102 of the input DTV signal from the ideal values of the PN511 signal 3201 and PN63 signal 3202, calculates the error for the data signal 3101 from the nearest value among the 8 kinds of fixed values, and outputs the error as the output error signal 1501S.
The prior art waveform equalization controller is constructed as described above and when this waveform equalization controller is used, it is possible that the waveform equalizer can be controlled and the DTV signal whose distortion is reduced can be obtained.
To be specific, the transmission line distortion of the input signal is estimated, and the learning control is performed so as to update the tap coefficient of the transversal filter that is included in the waveform equalizer in steps of a step size each time so that the distortion is cancelled out. Thereby, the signal whose transmission line distortion is reduced can be obtained as the output signal of the waveform equalizer.
However, the prior art waveform equalization controller has the following problems.
The first problem is that the step size, as the update step, of the tap coefficient is fixed. Generally, when the step size is increased, the follow-up property suffers in the case where the distortion is varied or the convergence speed at the initial operation is increased. On the other hand, the waveform equalization controller is easily affected by noises, and thus, the stability at the time of a low C/N is decreased. Conversely, when the step size is reduced, the controller is hard to be affected by the noises and the stability at the low C/N time is increased. However, when the distortion is varied, the follow-up property to the variation or the convergence speed of the waveform equalization operation at the initial operation is reduced.
The second problem is that two multipliers are required in the coefficient updating amount calculation unit to calculate the coefficient updating amount of one tap, and as a result, the circuit scale becomes larger.
The third problem is that when it is judged as “Stop” in the Stop&Go algorithm, the coefficient updating amount calculation unit 1902 outputs “0” up to the next iteration (update of the tap coefficient). Thus, the coefficient updating amount calculation unit 1902 is not utilized effectively as well as the follow-up property in the case where the distortion is varied or the convergence speed at the initial operation is reduced.